1. Field of the Invention
The present invention is directed to a method for making an electrical contact for a vitreous carbon electrode for subsequent connection to an electrical conductor.
1. Description of the Prior Art
Electrodes of vitreous carbon are suitable as implantable actuators and sensors in biomedicine (see German PS 26 13 072) Vitreous carbon electrodes have good biocompatibility and can have a high capacitance, and the cost for materials is low compared to other electrode materials. As used herein, the terms actuator or effector mean electrodes that have a stimulus effect, for example stimulation electrodes for heart pacemakers; as used herein the term sensor means an electrode with which measurements are made, for example an electrode for the detection of glucose.
Vitreous carbon electrodes are generally manufactured by producing an electrode blank composed of cross-linked artificial resin from a resin pre-condensate by curing, and this electrode blank is converted into the actual electrode by pyrolysis. Vitreous carbon is formed from the cross-linked artificial resin during the pyrolysis that generally ensues at a temperature between 700 and 2000.degree.C --in an inert atmosphere (see, for example, European Pat. 0 140 127). Electrode blanks can be brought into the desired shape relatively easily by mechanical processing; electrical contacting of vitreous carbon electrodes, however, is difficult.
Making an electrical contact for an implantable vitreous carbon electrode for attachment to a conductor, the vitreous carbon electrode having an electrode head and an electrode shaft, is generally accomplished by plugging or pressing a platinum sleeve onto the electrode shaft. Such a contact, however, is not only complicated to manufacture but also cannot be highly mechanically loaded. The diameter of the electrode shaft is limited due to design features of actuators or sensors, for example the need to mate with a hose-like insulating sleeve or jacket. Given an electrode diameter of 2.37 ram, for example, the diameter of the electrode shaft amounts to 1.34 mm for a heart pacemaker electrode and amounts to 1 mm for a measuring electrode of a catheter-like glucose sensor. Due to the mechanical properties of vitreous carbon, i.e. its brittleness, its Icad-withstanding capability with respect to tension and shearing is therefore extremely low given a contact of this type. For example, the tensile Icadability of the vitreous carbon electrode of a glucose sensor of the above-recited type only amounts to 280 p; with higher loading, the electrode shaft breaks. Such a contact can thus be easily destroyed by a shear fracture of the vitreous carbon during the manipulation of the electrodes, for example during assembly of the sensor. Moreover, further miniaturization of the sensor, which is usually desirable, cannot be achieved because the contact would then be even more sensitive.